The present invention generally relates to balloons for medical devices, as well as to processes and equipment for forming same. More particularly, the invention relates to medical or surgical balloons and to catheters incorporating them, such as those designed for angioplasty, valvuloplasty and urological uses and the like. The balloons exhibit the ability to be tailored to have expansion properties which are desired for a particular end use. The expansion property of particular interest is the amount or percentage of expansion or stretching beyond a non-distended inflated size at which the balloons are inflated to remove folding wrinkles but are not stretched. Balloons which are especially suitable in this regard are made of nylon or polyamide tubing that has been formed into the desired balloon configuration.
Catheter balloons and medical devices incorporating them are well-known for use in surgical contexts such as angioplasty procedures and other medical procedures during which narrowings or obstructions in blood vessels and other body passageways are altered in order to increase blood flow through the obstructed area of the blood vessel. For example, in a typical balloon angioplasty procedure, a partially occluded blood vessel lumen is enlarged through the use of a balloon catheter that is passed percutaneously by way of the arterial system to the site of the vascular obstruction. The balloon is then inflated to dilate the vessel lumen at the site of the obstruction.
Essentially, a balloon catheter is a thin, flexible length of tubing having a small inflatable balloon at a desired location along its length, such as at or near its tip. Balloon catheters are designed to be inserted into a body passageway such as the lumen of a blood vessel, a heart passageway, a urological passageway and the like, typically with fluoroscopic guidance.
In the past, medical device balloon materials have included balloons having a wall thickness at which the material exhibits strength and flexibility that allow inflation to a working diameter or designated initial dilation diameter which, once achieved, is not surpassable to any significant degree without balloon breakage or substantially increasing the risk of balloon breakage. Balloons of these materials can be characterized as being substantially non-distensible balloons that are not stretchable, expandable or compliant to a substantial extent beyond this working diameter. Such substantially non-distensible balloons can be characterized as being somewhat in the nature of paper bags which, once inflated to generally remove folding wrinkles, do not further inflate to any significant degree. Polymeric materials of this substantially non-distensible type that are used or proposed for use as medical balloons include polyethylene terephthalates.
Other types of materials, such as polyvinyl chlorides and cross-linked polyethylenes can be characterized as being distensible in that they grow in volume or stretch with increasing pressure until they break. These materials are generally elastic and/or stretchable. When such extensible materials are used as medical balloons, the working diameter or designated dilation diameter of the balloon can be exceeded, based upon the stretchability of the material.
Substantially non-distensible balloons have at times been considered to be advantageous because they will not inflate significantly beyond their respective designated dilation diameters, thereby minimizing possible over-inflation errors. The theory is that one need only select a balloon such as an angiographic balloon that has an opened and inflated diameter which substantially corresponds to the dilation size desired at the obstruction site. However, physiological vessels such as arteries are generally tapered and do not always coincide with readily available catheter balloon dimensions, and at times it may be preferable to be able to increase the diameter of the balloon beyond what had been initially anticipated. With a substantially non-distensible balloon, such further extension is severely limited. On the other hand, certain non-distensible materials out of which medical balloons are made generally possess relatively high tensile strength values, which is typically a desirable attribute, especially for dilating tough lesions.
More readily distensible materials such as polyvinyl chloride typically exhibit a lower tensile strength and a larger elongation than a substantially non-distensible material such as polyethylene terephthalate. This relatively low tensile strength increases the risk of possible balloon failure. Due to their larger ultimate elongation, most readily distensible materials can provide a wide range of effective inflation or dilation diameters for each particular balloon size because the inflated working profile of the balloon, once achieved, can be further expanded in order to effect additional dilation. But this very property of having an expanded dilation range is not without its dangers because of the increased risk of overinflation that can damage the blood vessel being treated, and the overinflation risk may have to be compensated for by using balloon wall thicknesses greater than might otherwise be desired.
Although a material such as polyethylene terephthalate is advantageous from the point of view of its especially high tensile strength and its tightly controllable inflation characteristics, it has undesirable properties in addition to its general non-distensibility. In some situations, biaxial orientation of polyethylene terephthalate will impart excessive crystallinity to an angioplasty balloon, or the Young's modulus will be simply too high. Under these circumstances, the balloon itself or the thicker leg sections thereof will not readily fold over and down in order to provide the type of low profile that is desirable for insertion through a guiding catheter and/or through the cardiovascular system and especially through the narrowed artery which is to be dilated. The resistance to folding, or "winging," is an especially difficult problem when it comes to larger balloons, such as those intended for valvuloplasty applications.
Also, it has been observed that thin-walled materials such as polyethylene terephthalate have a tendency to form pin holes or exhibit other signs of weakening, especially when flexed. Such a tendency can require extreme care in handling so as to avoid inadvertent damage that could substantially weaken a polyethylene terephthalate medical balloon. Although it is known that a lower profile and more flexible balloon and balloon legs can be made by thinning the wall, thus thinned polyethylene terephthalate balloons become extremely fragile and may not survive insertion through the cardiovascular system with desired integrity and without pin-holing and/or rupture.
Materials such as polyethylene terephthalate do not readily accept coating with drugs or lubricants, which can be desirable in many applications. Polyethylene terephthalate materials are also difficult to fuse, whether by adherence by heating or with known biocompatible adhesives.
By the present invention, these undesirable and/or disadvantageous properties of substantially non-distensible medical balloons such as those made from polyethylene terephthalate materials are significantly eliminated. At the same time, many of the advantages of these types of materials are provided or approximated. Furthermore, the present invention realizes many of the advantages of the more elastomeric materials such as polyvinyl chloride, but without the disadvantages of relatively low tensile strength, and the possibility of excessive expandability that can lead to overinflation of a blood vessel or the like.